Patent Document 1 (identified below) discloses a power supply device that includes a plurality of power supply units connected in parallel and that controls the number of operating power supply units in accordance with a load current. This power supply device can maintain high power supply conversion efficiency even in light-load states and achieve energy conservation by preventing a decrease in power conversion efficiency in light-load conditions.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2012-210013.
The power supply device in Patent Document 1 deactivates one or more power supply units during light-load conditions. If the load suddenly changes from light to heavy in the state where one or more power supply units are inactive, they may be unable to be quickly activated, and the power supply device may fail to support the suddenly changing load current. In this case, voltage supplied to the load decreases. In particular, if the load is an information processing system apparatus, such as a server, the voltage decrease may cause unstable operations. For this reason, the power supply device in Patent Document 1 is not suited for information processing system apparatuses, such as servers.